The present invention relates to an improvement of an electrostatic discharge protection circuit for a MOS device, particularly to an input circuit with a variable limiting threshold for preventing overvoltage from damaging a MOS-input semiconductor integrated circuit which is to be responsive to a high-voltage input signal.
Generally speaking, for an insulated-gate field effect transistor (MOS transistor) being formed with a metallic gate, insulation film and semiconductor elements, any countermeasure to avoid a faulty application of overvoltage to the gate electrode should be adapted. This is because such a faulty overvoltage application will rupture the electrical insulation of the gate insulation film and results in permanently damaging the MOS semiconductor device with a slight supply of power.
The gate oxide ruptures when the dielectric strength of the gate oxide, made of, e.g., silicon dioxide, is more than approximately 10.sup.7 V/cm. Thus, if a voltage of 30-50 V is applied across a 300-500 .ANG. thick gate oxide, a rupture occurs. It is conventionally known that such permanent damage can be prevented by adapting an input protection circuit which simply suppresses the peak potential of an input voltage below the voltage at which a rupture occurs.
However, when the above conventional input protection circuit is applied to an ultraviolet erasable non-volatile semiconductor memory (EPROM) or an electric erasable non-volatile semiconductor memory (EEPROM) and, if an input terminal of an EPROM or an EEPROM is designed to be commonly used for a low-voltage signal input and high-voltage signal input (whose potential will not cause said permanent damage), a certain problem arises. That is, when a high-voltage signal is supplied to the input terminal, the input protection circuit with a given fixed input limiting threshold could respond to the potential of a high-voltage input signal so that the necessary high-voltage component exceeding the input limiting threshold is suppressed or blocked. Such inconvenience cannot be avoided according to said conventional input protection circuit.